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What was the first flight instrument ever used in an airplane?
It's not magnetic compass, turn coordinator, stopwatch, wind meter, VVI, and airspeed indicator.
I want to know what is the first flight instrument.

That would probably be the engine rpm indicator.

According to this essay Orville Wright held a mechanical Anemometer and a stopwatch.

qq-jkztd I don't think so   because the pilot sense is not an inatroment

tachometer or watch

Rock. Indicates the direction of gravity.

I'd say a binary thermometer. As in does wax melt at this distance to the sun or not. Ikaros used one inefficiently.

@qqjkztd Mk Ⅰ Eyeball?

@coxy yes the mk 1 was the best, and it was usually installed as a redundant pair.   The other one would be a seat-of-pants sensor, an early form of bank angle, pitch, and inverted-flight detector.   This one worked best with a seatbelt!

It's the yaw string:

The yaw string dates from the earliest days of aviation, and actually
  was the first flight instrument.

Picture:

Source

Can you integrate some information from the article into your answer? What's a yaw string?

How did they use a Yaw string on planes without canopies?

@BruceWayne Slingsby T21 has no canopy and often has a yaw string tied to the pitot tube, which emerges from the top of the nose infront of the cockpit. Clearly, a pitot tube services an ASI, so this is perhaps a bad example!

LOL biplanes don't have yaw strings.

@BruceWayne: you tie the string to anything that protrudes.

@TylerDurden: that's why they slip so much in their turns.

The Wright Flyer, arguably the first controllable aircraft, had three instruments:
A stopwatch to measure air time.
A crude tachometer to gauge engine speed
And an anemometer, to make an estimate of distance traveled by measuring airflow.

The question does not specify powered flight, which this answer assumes.

@ErinAnne The question title does say "airplane," not just "aircraft," so that narrows it down quite a bit in regards to things that existed prior to the Wright flyers. It leaves only gliders and powered airplanes in consideration.

I'm pretty sure it had a yaw string as well-- as did several of the Wright gliders that preceded it.

I believe the correct answer (and the answer that you looking for) would be the forerunner of the of an artificial horizon, the yaw string. The tendency of airplanes (particularly pioneer aircraft) to roll needed to correct in a timely manner, and the visual earth horizon reference was not an effective means of detecting slip or skid in the necessary timely manner. Consisting of a suspended short piece string or yarn, often weighted the bottom give the pilot a quick measure of the side slip angle of the craft.
However, the magnetic compass in my opinion would be the first true instrument. Of the seven standard flight instruments, the altimeter, attitude indicator (artificial horizon), airspeed indicator, magnetic compass, heading indicator, turn and band indicator, and vertical speed indicator, the magnetic compass was not only an existing instrument, but would have become a natural necessity as flying evolved from the day-light, good weather over ground with recognizable landmarks, to night time, over water or monotonous terrain, or limited visibility, etc.
Early flight in conditions that disallowed landmark and horizon earth reference often used dead reckoning as a means of navigation. The ability to maintain a constant and correct direction of travel is essential to dead reckoning. A magnetic compass would have been the logical first step.
However, the earth magnetic fluctuations, mistaken speed of travel, and travel heading deviation (the sidewards travel of a vessel as the result of forces (such as wind, currents, etc.) makes dead reckoning as means navigation by any means of transportation, well, dicey. Consider 50 mile due south flight over water with no visual references, if a easterly cross wind deviates the flight path one half mile laterally for every ten miles of flights, then, even with a correct speed in relation to ground, the pilot could of the mark by two and half miles. Thus many other flight instruments had to be developed to bring aviation to where it's at today.
Known as Gyro, Lawrence B. Sperry, a well known for many innovative flight instruments he constantly conceived, developed and personally tested. Gyro was one the first to fly at night, regularly army night flights in 1916. Todays modern aircraft still equipped with the basic flight instruments the Sperry developed.


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Why does the Ikarus C42 pitch down when stalled?
Apparently, the C42 nose pitches down shortly after stall, like in this video. I know part of the video might be pilot input to recover from the stall, but I've read on various flying forums that this will happen naturally without input anyway. Why is this?

It's built in to the design of most aircraft as an inherent safety feature - one of the many reasons why flying is remarkably safe.

I might have to take my instructor up for another few fully developed stalls some time, but that pitch-down movement looks huge, making me think that the CG is quite far forward. What's shown in the video appears similar to what I did the first time we practiced stall recovery, at which time the instructor told me "don't dive". That's in a C42B, so not exactly a C42 but still a very similar aircraft. Really, at least the C42B I've flown has only needed about neutral elevator plus some engine power to recover from a fully developed stall with two adults onboard.

The Ikarus C42 is very similar to the Cessna 152 in design and power (but around 500 lbs lighter!).  This is a classic high wing design that is one of the safest aircraft one can fly.  Rock stable on all three axes and very forgiving.  These are a lot of fun to slip, as you will not see a lot of roll coupling with yaw (rudder) input.
As far as the video, this is a fairly normal stall recovery.  As with anything, practice makes perfect.
Properly weighted, this type of plane will not need a hugely aggressive push forward for stall recovery.  Just hold the nose straight with the rudder.
There are really 2 lines of defense in stall recovery with an aircraft properly balanced.  The first is a natural nose down with loss of speed caused by positioning CG ahead of center of lift from wing and trimming with downforce on tail (elevator up).  Because the trim torque is aerodynamic, it has less force when the plane slows down and the nose drops.  Planes set up this way will gently drop out of stall simply by relaxing the elevator.
The second line of defense comes from the surface area of the horizontal stabilizer (as seen from underneath the plane).  If the plane starts sinking in a nose up or level position, vertical motion will help flip the nose down sharply.  This is assisted with down elevator (stick forward).
The key is to hold the nose straight with the rudder.
Do not use the ailerons.  With the Ikarus, simply letting go of the stick should be enough to recover, but it would be highly recommended to take a lesson and learn the limitations and behavior of that plane.

Not a lot of roll coupling with rudder input in a slip in a Cessna 152?  I'd disagree on that one; the high wing placement creates a strong "dihedral effect".  If we want to talk about WHY that is so it would be good for someone to ask that as an actual question.

Have you flown one?  Do not understand your need to disagree.  These threads are gratis to all, let's not abuse them.  The reason why high wings don't couple roll with yaw is LOWER CG.  If you don't believe me, check in with the Antonovs.  I know what physics books are teaching, it ain't always  that way in the real world.  What Cessna has done is a masterpiece of force balancing.

Yes I have flown a Cessna 152 many times and I would say there is substantial coupling between slip and roll.  You've got to be joking if you are saying a high wing configuration has inherently less slip-roll coupling than a low wing, for the same actual physical dihedral angle.  Or that putting the CG way below the wing REDUCES slip-roll coupling.  Post it as a question if you want to discuss further and hear what other folks have to say on this.  Or just see https://www.av8n.com/how/htm/roll.html#sec-other-slip-roll .

Haven't you ever noticed that high-wing planes are usually designed with less actual dihedral than low-wing planes, for the same general class of aircraft (e.g. general aviation trainers)?  There is a reason for that.

I have flown a Cessna 172 and one can slam the rudder to the stop and it won't roll worth a damn.  That is what makes it so good in forward slips.

You are telling me that you don't need to hold any aileron pressure toward the low wing to hold the bank angle constant during a full-rudder slip?  You don't find a full-rudder slip to require cross-controlling to hold stabilized situation (constant heading and bank angle)?  It's no problem to maneuver a Cessna 152 or 172 through reversing turns of up to 30 degrees bank or more, with your hands completely off the yoke.  Just keep the roll rate modest to avoid a rapid rise of the nose as you come through wings-level and a rapid drop of the nose as the bank angle increases.

Yes, the reason is that low CG provides roll stability,  again see Antonov.  They actually have to put ANHEDRAL in to be able to roll it.  Sir, you have talent, so do others here.  Try listening.

Again, do a forward slip in a high wing.  That is one of their advantages.  You can work the ailerons BOTH ways.

See edit to my last comment.  What you are saying is 100% opposite to reality.

Roll stability is a DIRECT RESULT of slip-roll coupling.  That's where it comes from. The same characteristics that make a plane tend to be stable in hands-off flight, also make it respond strongly to rudder-induced sideslip.  See https://www.av8n.com/how/htm/roll.html#sec-roll-stability  OK enough on this.  Ask it as an actual question if wish to discuss further.

I don't need to ask it, nor do I need your clearly misguided arguments,  at higher speeds, aerodynamic effects are stronger (obviously). What concerns me most is your rather adolescent desire to impose your ideas.  That is YOUR way of seeing it.  Please consider others.

And PS, when considering "dihedral effect" in high wings, remember, the area BELOW the CG acts as a vertical fin (fuselage side area and landing gear) to lessen "dihedral effect".  It is not just the wing here.  Cessna knew  this and built a great airplane.  This is pure aerodynamic effect.  The low CG "myth" also helps keep it upright.

Two different consequences of high-wing configuration-- one, aerodynamic disturbance by fuselage of flow over wing as explored in link I already posted. Two, "parasol" effect that is present even if wing is 100 feet above fuselage so there is no aerodynamic disturbance-- here the wing is high above the center of mass of the whole aircraft, so in a sideslip, wing's drag component acts sideways relative to CG and creates a roll torque. Sure, of course surface area below the CG has the opposite effect. The fact remains in actual practice we see stronger slip-roll coupling with high-wing aircraft.

We should build models and try this.  I have come to realize the aerodynamic roll coupling force is lessened not by lowering CG, but by adding area UNDER the CG.  The "pendulum" only serves to RESTORE upright condition.  So we must consider side force on landing gear, fuselage, tail etc.  With large displacement of CG and lift (parachute) will tend to rock back and forth easily with side force.  Rather cleverly, putting the wing on top actually raises the CG, but puts more area underneath it.  I do not disagree aerodynamic force at wing/fuse will roll it.

Most aeroplanes are designed with the Centre of Gravity being ahead of Centre of Lift, so when the aeroplane's wing cannot produce sufficient lift anymore -- due to a high Angle of Attack, the nose will drop, therefore, it will decrease the AoA and as the gravity pulls the aeroplane towards earth, the speed will increase which in combination with the lower AoA will hopefully produce enough lift to keep the aeroplane flying again.
In some cases when the CG is not forward of CoL -- e.g. mostly in transport category aeroplanes, the horizontal stabiliser is set/installed on the neutral nose-down Angle of Incidence, which makes the aeroplane fly in a more stable manner and will help the stall recovery by pushing to nose-down.
Generally flying in any type of aeroplane with CG aft of CoL is considered a no-no, for the reasons explained above: 1) Flight instability and 2) possibly unrecoverable from a stall -- which in fact is likely to happen because of the displaced CG will tend to increase the AoA which will decrease the speed, until stall happens.

The second paragraph is incorrect. This configuration (CG is behind CoL of the wing (which is usually called Centre of Pressure, CP)) is less stable and doesn't really help stall recovery. When it becomes actually unstable (CG behind the Neutral Point, NP), the tail will have to be set at a higher AoA than the wing (generally), which means it will likely stall first, causing severe nose-up effect. In general, flight stability is related to NP, not CP.

Your psychics are a bit off here. To obtain a balance with three forces either all three forces must pass through the same point or they must be parallel with the middle force in the opposite direction of the other two, and the ratio between the distances of the outer forces from the middle force must be the inverse of ratio of the magnitudes of the outer forces.

Also, your description of why the plane noses down in a stall is wrong. It is not about where the center of gravity and center of lift are positioned, but rather about the relative movement between the two.

Most (if not all) longitudinally stable aircraft will pitch nose down after a stall.
This is because the forward flying surface (regardless of conventional or canard layout) -- or forward portion of the wing, in the case of tailless designs -- must fly at a higher loading and coefficient of lift than the rear in order to maintain stability, so when lift is lost, it will be lost first at the higher-loaded and higher-coefficient surface, which will then start to drop before the lower-loaded or lower-coefficient surface.

Unless I'm misunderstanding your answer it doesn't seem correct. Stall generally starts at the rear of the wing and progresses forward.

@TomMcW, stall starts at the trailing edge and progresses forward, but the answer says the forward airfoil stalls before the aft airfoil. Tailless designs are always (at least a bit) swept, so the root is the forward airfoil and the tips are the aft airfoil and the twist ensures the proper order of stalling. Untwisted swept wing would stall tip-first, which would cause pitch up, but that is why most swept wings are twisted.

@JanHudec Non-swept tailless designs exist (Flying Plank, for instance); they depend on reflex airfoils to provide pitch stability.  This keeps the aft portion lifting after the leading portion stalls, so they, too, pitch down after stalling.  If they didn't, they'd be death traps.

@ZeissIkon, interesting, I didn't see that.

@JanHudec First, I'm not sure why that part is included. The C42 is not tailless, nor does it have canards or swept wings. That said, it's the phrase "forward" and "aft" airfoil that threw me off. I get what he means now, the root is forward of the tips in a swept wing. It's just confusing wording and really doesn't pertain to the aircraft in question.

@TomMcW, the part is included because it explains that the behaviour is universal. And forward and aft airfoil are somewhat common terms when discussing stability requirements.

I guess if it said forward and aft *portion" of the airfoil it would be less confusing. But in the situation at hand, an unswept wing, a twisted airfoil would be intended to maintain aileron authority and not contribute to the pitch down effect. That would be entirely due to the center of lift being aft of the cog. Am I incorrect?

@TomMcW Whether in the simple case of conventional layout, no sweep, or the extreme of a no-sweep tailless, the CL is never actually aft of the CG.  The combined CL of the entire lifting surface -- wing, stabilizer, third surface if present, fuselage, and external payload if any -- must coincide with the CG in steady state flight, else you'll have a pitching moment.  The tendency to nose up with speed increase is pitch stability, and it requires something forward to have a higher coefficient than things aft -- else your airframe is unstable.

@ZeissIkon: From what I've read, the original Flyer was deliberately designed so that it wouldn't pitch down, nor be able to regain airspeed, if it stalled.  This didn't make it a death trap, however, since it would descend with a largely uncontrollable but safe velocity and attitude.  Such a design wouldn't be suitable for e.g. flying over buildings or water, but had its advantages back in the day.

@ZeissIkon I'm still confused, but I don't want to string out the comments. If you feel like it, hit me up on chat

@supercat As I understand it, the Wrights built the 1903 Flyer to be marginally unstable because they weren't certain they could control it if it was "too stable".  After they found out how unstable it was, they built the next model with a reasonable stability margin.  The Flyer wasn't a deathtrap only because it flew at 20-30 feet, at 35 mph, instead of a few hundred feet at 70 mph.

@TomMcW I can't chat now, but I recommend reading up on design for stability, especially as relates to model airplanes.  With no pilot aboard, they must typify stability.  If you can find it, Keith Laumer's model airplane book is quite good.

@ZeissIkon: I don't think terminal velocity for an object of the Flyer's shape and weight would be very high if it maintained an attitude with the wings roughly horizontal.  The first flying fatality occurred in a plane which had a control cable break, but I'm sure there had been many stall-induced crashes before that, from altitudes far greater than 20-30 feet.

@supercat The Flyer was heavier than it looked -- 600+ lb dry, or a bit more than double what a Part 103 ultralight the same size would weigh.  Much of that weight was the engine.  Terminal velocity in a deep stall would have been plenty to kill, especially with 200+ lb of engine sitting beside you and a structure that would collapse on impact.

@ZeissIkon: Hmm... I forget the title of the coffee-table book I'd read about it, but it had seemed authoritative.

As a stall is approached, the center of lift moves slowly forward. The pilot will compensate for this with either back-pressure on the stick/yoke or by trimming the aircraft. When the stall occurs the center of lift suddenly and rapidly moves towards the rear of the aircraft. Since the aircraft was balanced before, moving the force of lift backwards will cause the rear of the aircraft to rotate upwards, and the nose to drop. In a traditional aircraft configuration, the wing will stall before the horizontal stabilizer. The horizontal stabilizer normally produces lift in the downwards direction just incase it does stall, that way the nose would automatically drop. Canard configurations achieve the same effect by having the horizontal stabilizer produce positive lift and stall before the wing.

I don't understand how the center of lift moving forward would cause a nose-down moment.

It is the rapid movement of the Center of loft to the rear that causes the nose to drop. The forward movement earlier on may or may not affect trim depending on how slowly the aircraft is brought to the stall as well as different design characteristics pertaining to the wing and horizontal stabilizer.

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Why did Boeing keep using yokes while Airbus moved on to sidestick?
The first Airbus had yokes but they changed to the sidesticks after a while But Boeing is still stuck with yokes even with 777-300ER decades later.
The sole purpose of the yokes are seemingly for the Cessna students to stick their charts and for some instructions on the airliners which I think isnt worth blocking the screenes.
One more thing. Is it true that its impossible to manual reversion an sidestick aircraft?
Why though? Boeing simply wants to keep R&D low or there are some other reasons? They are known for using the same design to keep costs down but didn't they ever tried to innovate the mechanics?
Perhaps just another example of design philosophy between the west and the east?

"The sole purpose of the yokes are seemingly for the Cessna students to stick their charts " -- wow-- is this a question, or a rant?

@quietflyer I just couldn't think of a better advantage of yokes. They seemed to be perfect for putting your tablet or navigaton charts and easily access them without putting them over the windshield or put on the kneeboards on your legs.

Sidestick controllers for the 737 have been evaluated (at least by NASA), and there was some disagreement with this being too major a change from older airliners. The 737 with its high-bypass engines and design that has almost stood up to this day was a revolutionary aircraft already.
Airbus was entering their A320 into the market in 1987, a full 20 years later than the 737. Before the A320, if you wanted the most advanced large narrow-body, it was - technically, it was the 757, but since you didn't want to pay the cost and weight premium it commanded, and narrowbodies mostly serve shorter routes - it was the 737.
This meant Airbus needed a disruptive design with killer features to have any shot at the market. So they gave the A320 the most sophisticated fly-by-wire system, convenient sidestick control, and a ULD-supporting cargo hold. There was less concern about commonality as it was an all-new type anyway.
Both got what they wanted. Boeing got, with the 737, an airplane that was easier to retrain 707 pilots for. Airbus went from minimal market share to roughly equal footing with Boeing by differentiating their offering with the A320.
As time went on, both kept playing to their strengths. Boeing wanted to keep retraining pilots between their models easier, so they kept the yoke. There's still a lot of innovation in the background, but the pilot-perceived change is reduced when possible. Airbus kept developing their glass cockpits with further reduced pilot effort.

Boeing also has a different basic philosophy with respect to the pilot's place in the airplane, tending to keep the pilot deeper in the control loop you might say, with a bit more authority over everything, than Airbus, and their pilots had more design influence within development programs.  And they tended to dislike side sticks.

@ymb1 I'll have to check for better sources, edited it to be less-definitive in the meantime.

There are very good reasons to implement coupled column/yokes for aeroplane control: they are right in front of you, you can apply force with both hands, and you can feel immediately what input the other crew member is giving. The force sensors in our hands are very precise feedback instruments that should and can be fully utilised.
The upside of the side stick for passenger aircraft is...actually, what was the problem that needed to be solved? More room for the lunch table?
The F16 used side sticks because of the 9g accelerations that accelerate arms all over the place and can be the source of Pilot Induced Oscillations - best to support the arm with the armrest and control the aircraft by wrist movement. Not a lot of movement because the sticks are almost fixed, and use the force sensors in the hands for pilot feedback.
Column/yoke combinations provide the clearest feedback on what the condition of the aeroplane is: hydraulics ON, autopilot ON, trim position. The 737 is about the largest plane that can handle manual reversion, and one requires a column/yoke or a centre stick for that.

Boeing continues to use columns on their full FBW designs because that's what their flight ops contingent, and a lot of pilots generally prefer, and they feel strongly enough about it to live with the 150lb or so weight penalty of the columns, torque tube, etc, even though there are no control cables.  On the C series program there was a long debate over whether to go in the Airbus or Boeing direction, and the decision was made to use side sticks for the weight saving.

Not only can you use a yoke with both hands, you can use it with either hand.  If you use a sidestick, you'd have to have one on either side of the pilot to get the same capability.

This is a very old discussion I have heard many arguments from Boeing and Airbus.
In my airline, old A340 pilots assigned to fly B787 really misses the sidestick. In the other hand, Boing pilos assigned Airbus feels confortable with sidestick very fast.

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Gage or Gauge? Which is correct in the aviation industry?
Gage or Gauge?
Some official aviation sources seem to use the spelling "gage" whether referencing cockpit instruments or aircraft maintenance tooling.
However, more broad English sources note that "gage" is either interchangeable with "guage," or flat out incorrect.
Is either spelling technically correct, or only one?
Do the different spellings hold different meanings or connotations in aviation?
Are there official examples that could make the argument "gage" could be correct in specific circumstances (patents, manuals, industry standards, trademarks)?
Is there a history in early aviation using "gage" that has been carried over unofficially to today (like how older aircraft operating today may have mph on their airspeed indicator)?

What's the context?

You may get a better response on english.SE. Identifying the correct spelling of a word isn't very aviation-related, IMO, even if the word in question is.

"I see gage used rampantly in aviation" I'd be interested to know where.

"Some official aviation sources" Which ones? Please give a few specific examples.

I think this should be reopened.  This is not unlike asking is it more prevalent to say "aeroplane" or "airplane".  Some things aren't wrong or right.  It could be a nationality issue like 'aluminum' vs. 'aluminium', but since the question was voted to close, now none of us who don't know will learn the answer.

@RyanMortensen The question might have merit, but it'd probably be a lot easier to answer if OP just provides a few examples where (in this case) "gage" is used in aviation. As it stands, I think this is asking for us to guess at a little too much.

OP: the edit has not helped you, as now you're asking too many questions all at once. And this is still a better fit for [english.se]

Gauge is correct. As in Oil temperature gauge, Fuel Pressure gauge, Fuel Level gauge, Cylinder Head Temperature gauge, Exhaust Gas Temperature gauge, etc.
Anyone use gage is misspelling the word.  Same with Hangar. (Not hanger)

To clarify, both "gage" and "hanger" are real words, but not as likely to be used in aviation (especially "gage")

@fooot I don't know; I might appreciate some kind of hanger in the cockpit of an airliner, to hang my coat on; especially during the winter months...

Yes, but not for parking your airplane in so you can work on the gauges.


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How does thrust reversal work on a jet engine, and when should it be used?
A two part question this time, though they are closely related enough that I figured I'd knock them out all at once.  I figure the first question probably informs the second question anyway.
Firstly, how does a jet engine flip over to reverse thrust?  I'm assuming the turbine doesn't actually switch directions.  Is there a set of blades towards the rear of the engine that divert the flow forward again?  How's that work?
Probably more topically:  When is the right time to apply reverse thrust?  I'm assuming you ought to be on the ground (that seems like a given), but are there checks that a pilot has to do before engaging the reverse thrust?  Are there times when it's not used, or when it possibly just isn't necessary?

I can hardly wait to see a jet engine "flip" the next time I go flying...

@jwzumwalt It’s an idiomatic phrase, it simply means “change too” and is a reference to how mechanical changes are often actuated by the flip of a switch.

The engine does not move, the air flow is only redirected. The method of redirecting the flow varies with the size, configuration, and manufacturer of the engine. The flow does not need to be directed completely forward; the flow is generally mostly outwards and partially forward. This is still enough to create significant drag and slow the aircraft down.
In smaller and older engines, the entire flow is redirected. This corresponds to the first two configurations below. Buckets or clamshell doors close over the jet flow to redirect the air.
In larger engines, particularly high-bypass turbofans, only part of the air is redirected. This corresponds to the last configuration below. The core air from the engine still exits normally, but the bypass air from the fan is redirected. Since the bypass air on these engines is a much greater flow than the engine core air, this results in a net reverse thrust. There are multiple mechanisms, but the general idea is to deploy doors that block the bypass air and send it through the sides of the engine cowl.
Turboprop aircraft just change the pitch of the propeller blades such that the propeller pushes air forward instead of backward. The pitch in which the blades provide reverse thrust is called the "beta range."

In civil aircraft, reverse thrust is only used when on the ground. There are typically interlocks in the system that prevent the thrust reversers from deploying if the aircraft does not sense that it is on the ground. Once the aircraft touches down, the pilot will deploy the reverse thrust. Some military aircraft like the C-17 can use reverse thrust in the air. This enables them to perform very steep descents.
Reverse thrust is more of an "optional" method of braking (see this related question), only to provide extra stopping power when needed. This particularly helps when braking force is less effective, such as in rain or snow. Before landing, pilots will take into account the wind, aircraft weight, runway length, and any contamination of the runway (rain or snow). Based on this they will know what kind of braking force is needed and whether they should use thrust reversers.
The thrust reversers don't always have to be operational for an aircraft to fly. Some operators choose to disable them, which decreases maintenance costs. The thrust reversers can also break, in which case they will be mechanically locked to prevent them from deploying until they can be fixed. Pilots will consider this when they are computing their landing distance.

And here's one example of what happens when the reverse is broke and mec. locked but the pilot misunderstands what to do with thrust levers: http://en.wikipedia.org/wiki/TAM_Airlines_Flight_3054#Crash

Thrust reverser: There are a number of types, ranging from bucket reversers that swing a pair of doors into the exhaust stream, directing it forward, to door-based systems that direct the bypass air of a high-bypass turbofan engine out the sides of the engine in a somewhat forward direction.
In most cases, you want to apply reverse thrust as soon as the wheels are on the ground.  Some aircraft can use in-flight thrust reversal for a quick reduction of speed or altitude, but these tend to be military aircraft that need the improved performance.  You want to stop using reverse thrust as soon as you're going slowly enough that any debris kicked up could make its way into the engine intake, which is why airplanes mostly don't use reverse thrust for backing up.
As a special case, turboprop engines can twist the propeller blades so that the normal rotation of the propeller pushes air forwards rather than back, but you may not consider these to be "jet" engines.

The DC-8 variant that was re-engined with turbofans was certified to use reverse thrust in flight.  NYC-LAX flights did use RT for exactly the reasons mentioned by Mark.  The flight deck always came on the intercom before entering reverse thrust to warn the pax of an unusual noise and buffeting while this was in operation.

I don't have anything to add to the two excellent answers above about how thrust reversal actually works but one really interesting application of it when not on the ground was in NASA's Space Shuttle Training Aircraft, a Grumman Golfstream II which was modified to have the same handling characteristics and approach profile as NASA's Space Shuttles.
To match the descent rate and drag profile of the real shuttles the landing gear was lowered and reverse thrust was engaged mid-flight and controlled by a purpose built system.

Apparently the system was incredibly realistic and each pilot and commander carried out about 1000 flights before their one shot landing the real thing. It even had the same control surfaces as the shuttle on one half of the cockpit:

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How do you hold at a non-towered airport?
Let's say, hypothetically, that you enter the pattern for a small, non-towered airport (my local one, 3DW, may be a good example.)  But as you get into the pattern you find out that the runway is occupied by another craft and, for whatever reason the runway will probably be occupied for another 20 minutes, much longer than it will take you to complete the pattern.  So you'll have to delay landing.
But you've already entered the pattern, so what should you do?  Is there a standard holding pattern for small airports that incorporates parts of the pattern (maybe some little loop where the base leg and downwind meet)?  Or do you just fly away form the airport a bit and make up your own holding pattern?

I don't know the answer but; you hope that everyone is making position calls.  You hope that they are accurate (are they calling downwind abeam the threshold or mid-runway, or just after turning downwind?).  You hope that everyone knows the procedure.  You hope that you will only do one or two orbits and hope that no-one else joins the pattern in the meantime.  All of those hopes, perhaps multiplied 2 or 3 times as you fly around, suggest that regardless of some official pattern, flying someone quiet and nearby to make up your own hold might be prudent.

No well behaved craft should hold a runway for 20 minutes.  If a plane is not exiting a runway, they shouldn't be on the ground any longer than a stop-n-go, or perhaps a back-taxi on the runway, which should still be relatively short.

@abelenky I once landed with a flat tire that stopped me dead in the middle of the runway at a non-towered airport. I was stuck there for about 20 minutes until they could get a tug to drag me off. The airport was closed during that time. So it happens.

"... you find out, via radio ..." Then presumably you mean that you found out from the pilot on the runway? Or are you saying you found out over the radio from someone else? If you found out from the plane on the runway, then why not ask him how long he plans to be there? If he's gone in a few minutes, you could linger around, and if he has a heart attack, perhaps you could HELP by informing ATC over the radio to send an ambulance? Why not ask?

There's no "standard holding pattern" at any airport, towered or not. If the runway is occupied or temporarily closed for some reason, you might was well leave the traffic pattern and go putt around for a while someplace where there are fewer aircraft. At a towered airport, the controller will likely tell you what to do - but again, you can make both his life and yours easier by just leaving the pattern and going somewhere else until the situation is resolved.

Given that you want to stay in the pattern you could use the upwind leg to avoid flying over the runway.  This is, from base go to upwind, then crosswind, etc.
As others posted there is no standard holding pattern.  For VFR traffic there are no holds either.
The pattern may become congested quickly.  Your best bet is to divert until the situation improves.

As far as I know, there's no such thing as a "standard holding pattern" for uncontrolled fields, though I suppose Center might put you in some holding pattern if you're IFR. For VFR, though, if I found out that the runway was going to be unavailable for 20 minutes (and was reasonably likely to be open again after that time and I had plenty of fuel,) I'd just leave the pattern and fly a few miles away from the airport for a while. I'd then continue to monitor the CTAF to hear what's going on and if/when the runway is open again. If it seemed like it was going to be a while, I'd just go land at another nearby airport.
Most of the time, however, delays at uncontrolled fields will be brief (i.e. someone is back-taxiing, is taking off, is on final, or just landed) and you can just either make a 360 or a 270 degree turn to base (to the opposite direction as normal) to waste time until the runway is clear. Normally, this won't be more than a couple of minutes at a time. Normal checks, configuration changes, etc. should be made before entering the runway, so you won't normally encounter longer delays unless there's an actual problem of some sort.

Could you just fly in a circle at the start of the base leg for a couple minutes as well?  I'm guessing so long as everyone is talking that's not a big deal right?  I mean, if it was 20 minutes you might as well fly off, but if it was going to be, like, 5 minutes, maybe just do a couple circles at the entry to the base?

@JayCarr Yes, like I said, you can just fly past the base leg, then turn the opposite direction of the pattern in a 270-degree turn that will have you end up on the base leg. If the runway still isn't clear by the time you're about to enter the base leg, you can just keep turning and do another 360. In a standard rate turn, a 270 degree turn takes 1.5 minutes and a 360 followed by a 270 would take 3.5 minutes, which is more than plenty time under most circumstances.

As a matter of procedure I'm not a fan of making spacing turns in the pattern at uncontrolled fields, especially if there's other traffic around (there's no guarantee they're aware of what your'e doing - remember some of them might not even have radios). Also at least in the US there's the small matter of 91.126 "All turns must be made to the left" (or approved direction per the segmented circle) when operating in the vicinity of an (uncontrolled) airport. If something happens that disrupts your traffic pattern it may be better to break out (climb away) and come back to rejoin the pattern.

@voretaq7 Hmm... That rule is particularly interesting, since it directly contradicts the AIM's instructions to enter the pattern at a 45 degree angle (which requires a turn opposite the pattern to turn on to the downwind.)

@reirab Asking for internal consistency from the FAA? You must be new here! :)

In order to answer this question the terminology and procedures must be clarified.
If an airplane is in the airport traffic pattern and the runway won't be available by the time by the time it would reach it, the airplane can extend the traffic pattern to give the runway time to be available for landing. If it's going to take 20 minutes the airplane would need to depart the pattern and return later.
The use of the word hold is out of context here. As defined by the Instrument Flying Handbook:

Holding is a predetermined maneuver which keeps aircraft within a specified airspace while awaiting further clearance from ATC.

The way the question is asked it reveals that the person asking assumes that a hold is something performed in an airport traffic pattern, but it is not. A hold would be used at an enroute altitude and outside the immediate terminal aerodrome environment where traffic patterns are flown.
If a runway were unavailable for 20 minutes and an airplane were enroute on an IFR flight plan, ATC might issue that airplane a hold which is published on an instrument approach procedure for that airport. The airplane might hold there for 20 minutes until the runway became available again. This scenario would happen before the airplane reached the traffic pattern or after it departed it.

A holding pattern just means circling the airport. If you had to wait there, that is what you would do, circle the airport. If there are so many aircraft, that the pattern altitude is full, then you start stacking 2000 feet, 3000 feet, 4000 feet, etc.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What does "clear of clouds" mean in FAA FAR part 103?
In FAR part 103 when they are talking about visibility requirements for flying an ultralight, sometimes instead of giving exact distances, they will just say "clear of clouds".
For example:
                                   Minimum
                                  flight
                                visibility
      Flight altitudes             /1/       Minimum distance from clouds
1,200 feet or less above the
surface regardless of MSL
altitude:
(1) Within controlled airspace      3       500 feet below, 1,000 feet above, 2,000 feet horizontal.
(2) Outside controlled airspace     1       Clear of clouds.

Or here:
                                      Flight
          Airspace                visibility        Distance from clouds

Class G:
1,200 feet or less above the
surface (regardless of MSL
altitude)                       1 statute mile   Clear of clouds.
More than 1,200 feet above the
surface but less than 10,000
feet MSL                        1 statute mile   500 feet below.
                                                 1,000 feet above.
                                                 2,000 feet horizontal.

What does "Clear of Clouds" mean in this context?  Just that I should remain clear of the clouds, or does it mean that the sky must be clear of clouds in the defined space?
And if it does just mean I need to stay "clear of the clouds", how exactly is that defined?  1000ft?  100ft? 10ft?  What exactly?

keep clear of them. don't go in them.

It means you have to stay outside the cloud while flying. You're expected to use your own good judgment (remember it is illegal to fly into the cloud, and if you don't have proper instruments and know what you're doing, it's downright deadly) but there is no minimum clearance as there is at other times, so if you really felt like a daredevil you could reach out and touch a low-hanging "popcorn cloud" with your wingtip. This term is often used in situations where vertical separation minima from clouds is not practical (to stay 500 feet underneath a VMC-friendly cloud layer would require altitudes as low as 500 feet AGL; there are a lot of hazards to navigation at that height from radio antennae to city skylines)

The term also shows up in controlled airspace sometimes (Class B airspace, when you're under positive control of ATC, requires VFR traffic to remain "clear of clouds" ; Class C/D airspace when operating on a Special VFR clearance reduces the requirement to "clear of clouds" as well).

Edited; the text in the OP includes an old section no longer in effect which required ultralights to simply remain "clear of clouds" at less than 1,200 feet in uncontrolled airspace and a standard minima within; the new regs read slightly differently.

If you link me the new regs I'll substitute it in, I was having a hard time finding the official ones for some reason, and then I made an assumption about the accuracy of a non official site that was, well, wrong...

The document you linked to has the latest regs, but also has the older ones for reference. Here's the most recent I can find last amended 2014: http://www.ushpa.aero/documents/sop/sop-12-04.pdf

Here's the very latest from the FAA's EFCR site: http://www.ecfr.gov/cgi-bin/text-idx?SID=9440e72c3266431f250d0a14b3f7e04e&mc=true&tpl=/ecfrbrowse/Title14/14cfr103_main_02.tpl

Helicopters often fly "clear of clouds", reference 91.155

Clear of clouds means you should not fly through clouds. You may get as near as comfortable to clouds, but flying into a cloud is still prohibited. There is no definition with a particular distance that I am aware of.


text:
How do manufacturers engineer their aircraft to deal with tail strikes?
I have been told that tail strikes on take off and landing are rare but do occur.  I think, in my mind, the tail touching the ground at all would just make the tail fall off, but it appears I was wrong.  In most of cases of tail strikes I've read about the plane is able to either continue on it's journey or can simply circle back to the field it departed from.
So I'm wondering, what do big commercial aircraft manufacturers (like Boeing and Airbus) do to keep tail strikes from turning into a major disaster?  How do they engineer the plane to survive the incident?

Do you have a source that supports tail strikes being common?  I would characterize them as quite rare.

@casey I actually really struggled with the wording for that part.  All I'm trying to say is that it happens more often than "never", and it doesn't always involve major damage the the aircraft in common...  In the end though I decided to just leave it because it's not really a part of the question, it's just a set up.  But if you have any ideas on clarifying it...

There are a number of hull types for planes.  They include: truss, geodesic, monocoque, and semi-monocoque.  Basically what you're asking is "Why don't tail strikes destroy the hull of a plane?"  The answer is that each of these hull types provides some resistance to that type of blunt force.  I don't have any actual sources to cite, but I would guess that the order of "most tolerance" for tail strikes would probably be:  semi-monocoque, geodesic, monocoque, and then truss.  However, that's just an educated guess based on their general designs.

@Calphool That is at least the start of an answer though.  I do know they also specifically engineer the tail section to deal with the force of a tail strike, sometimes adding skid plates or even a small wheel...  But I've got scant details, so I'm hoping someone can step in and offer a more complete answer.

I've watched thousands of takeoffs and landings and only ever seen 1 tailstrike. And that one was during an airshow when an F-15 pilot decided to do something spectacular and pulled up way too early on takeoff. He hit his tailpipes (both of them) on the asphalt, damaging his aircraft severely (I think he damaged the runway as well). So no, they're not at all common.

@jwenting Again, it is a problem of wording, not my understanding. If you have a useful edit, please feel free.

AFAIK, tailstrikes normally mean the plane needs repairs. It can't just take it without damage. JAL123 was ultimately caused by tailstrike damage (the repair perfomred was performed badly, but the basic issue was a crack caused by the tailstrike).

@jwenting There, I think that wording is better.  I found an article on the WSJ that makes it sound like they happen at a rate of roughly once per year...  Having a devil of a time finding hard numbers though:  http://247wallst.com/special-report/2013/04/24/the-nine-most-common-airplane-accidents/

Though I did just find another article that says tail strikes happen at least 3 times a year to German aircraft (according the German investigators...).  >< I don't like this lack of hard data...  http://www.flyertalk.com/the-gate/blog/18769-the-tale-of-the-tail-strike-damaged-aircraft-crosses-ocean-—-and-no-one-realized-it.html

But the overall point is that they happen, and I wonder how the damage is kept to a minimum.

Sometimes the damage is severe...http://www.youtube.com/watch?v=C74ZXW2NlSY&search=airplanes

I think the main engineering solution is that the bottom of the tail is curved inwards. If the under side is flat like in smaller aircrafts, tail strike would be a lot more common. Testing then verifies that the parameters used to calculate the rotation speed at take off gives sufficient vertical speed per rotation to avoid tail strikes. If this speed can't be achieved given the targeted runways length, then it's back to square one, adding more engine power, bigger wheels, more curve, wing designs to improve lifts, or by reducing the maximum rotation during take off. Engineering is iterative.

"the tail touching the ground at all would just make the tail fall off..." What, like the ship incident where the front fell off?

During testing, the rear fuselage is protected by a beam made of oak or even steel to distribute the tail strike loads and to protect the aircraft skin.

A340 during tail strike tests. This one is literally blazing along the runway …
For normal operations, the protection is removed. However, tail strikes must not cause flight-critical damage, so a few precautions are taken during design:

No hydraulic or electrical lines must run along the bottom of the rear fuselage.
If there are still mechanical control elements like pushrods, they and their collapsing supports must not be close to the bottom of the fuselage.
The tail strike location must be outside of the pressurized part of the fuselage.

The main protection against tail strikes, however, is procedural: The rotation speed must be high enough to allow lift-off before a critical pitch attitude is reached.
Delta wing aircraft are much easier to over-rotate, so the Concorde used a retractable tailwheel for tail strike protection:

I like how the A340 says "smoother" on the tail...with sparks flying off of it.

Doesn't Airbus know that it's rude to set a fire under your airliner's tail?  :-P

The 777-300ER locks the gear bogey in the horizontal position during takeoff making it considerably more difficult to tailstrike (Boeing said they hit the ground 12 times during testing with the -300, but only got within 18 inches on the -300ER).

Source
See these documents: 1, 2

Firstly, this article from Boeing explains how tail strikes occur.
And
this Boeing informational note gives more practical information on tail strikes and prevention:
A short excerpt from that note:
"...some 777 models incorporate a tail strike protection system that uses a combination of software and hardware to protect the airplane. And some models of the 737, 767, and 777 have a tail skid that prevents damage from most takeoff tail strikes. However, these devices do not guarantee protection for landing tail strikes and some takeoff tail strikes. They also reduce tail clearance distances."


text:
Why are swept wings better for breaking the sound barrier?
I'm told that swept wings perform better when an aircraft is trying to break the sound barrier.  I was wondering why that would be?

Ad note: X-1 wing is not swept at all. It is "tapered", with swept leading edge and back-swept trailing edge, but the line of maximum thickness is either not swept or swept so little that it has no practical effect on supersoninc performance.

Sweep did not fix the control problems. This question should really be split in two, one for the sweep and one for the control problems.

@PeterKämpf - Okay, I'll split this up then.

@PeterKämpf There we go: http://aviation.stackexchange.com/questions/5003/what-control-issues-did-the-x-1-have-to-overcome-to-achieve-supersonic-flight

It's all about the 'effective' Mach number that the wing sees.  In essence, for a swept wing, the Mach number that the wing 'sees' as far as compressibility effects are concerned is the Mach number of the component of the freestream normal to the wing chord:

(Source: stanford.edu via archive.org)
By sweeping the wing back, you can get closer to the speed of sound without getting supersonic flow over the wing (remember, a wing generates lift by accelerating the air flowing over it, thereby decreasing the pressure - so you'll get supersonic flow over the wing before the freestream is supersonic).  Of course, once you fly supersonic, there's really no getting around having shock waves over the wing - however, you can reduce their strength with wing sweep.

Note: See this question for egid's answer (referenced above) to the removed portion of the question.

Swept wings are beneficial at transonic flight regimes (M=0.8-1.2). At transonic flight regimes there is a drastic increase in drag (CD is more or less constant up until that point) due to the effects of compressibility, which manifest themselves in local sonic regions on the wing (the plane itself can be flying at M=0.78, yet over some portions of the wing, where the pressure is low, velocities may go sonic and supersonic). There is a very large amount of wave drag in such regions. The swept wing essentially reduces the effective velocity, so the flow remains subsonic and shocks do not form over the wing, thus not leading to the generation of large amounts of drag. Check out stuff on critical Mach number (drag divergence number).

Jacob's and Flanker's answers are correct. That's effective Mach Number. As I remember it's been detected by Busemann.
Swept wings can fly therefore sub-sonic depending on Mach number.
OK- one can fly also with straight wings supersonic- then one needs other airfoils. See e.g. the starfighter. This is a non-swept-design because Starfighter is an american design and Busemann was a german.
Disadvantage of sweep: One has boundary layer displacement and therefore it's nearly impossible to keep laminar flow.
Advantage of sweep: Reduced motion of neutral point compared to straight wing when going to supersonic. Therefore some machines with straight wings crashed because pilots couldn't recover.
----Edit ---
Sweep reduces motion of the neutral point. Lowest motion is visible for a delta wing. : See Schlichting/Truckenbrodt Band 2 P. 214/227 ch. 8 pict 8.59/8.70 (my bible).
Therefore sweep/delta reduces control problems, sorry.

the F-104 has a swept leading edge, though, not strictly "straight wings". Also, please link the answers or mention the authors, saying "Answer 1 +2" is not really helpful.

@Federico Yes and no. The sweep is minor - it's a supersonic sharp leading edge.  In supersonic flow neutral point moves from 0.25 (subs.)  to 0.5 (supers.) when I remember correctly. Therefore for an aerodynamicist in supersonic flow it's nearly a straight wing.
Sorry, my first answer- wasn't aware that they are shifted.

In general, swept back wings aren't better at breaking the sound barrier. Indeed, if you see the first plane to break the sound barrier in horizontal flight (the Bell X-1) it didn't have swept wings. And it was developed after swept wings were developed.
What happens to wings as you get supersonic is shockwaves flowing over the wings. Specifically control surfaces. If you get shockwaves over your control surfaces you will basically lose control.
During WW2 several P-51 Mustangs and P-38 Lightnings crashed after being unable to recover from a dive. Both planes had engines fast enough to bring them to 70% the speed of sound in horizontal flight. In a dive it is entirely possible for them to break the sound barrier. And indeed it was speculated that that's what caused the crashes. Dive breaks were retrofitted to P-38s to allow them to slow down enough in a dive to regain control.
If you want to continue flying at or beyond Mach 1 you need to avoid your control surfaces from entering shockwaves. Since shockwaves are generally conical in nature you basically have two techniques you can use:

Sweep the wing back so that they stay within the cone of the shockwave.
Make your nose longer so that you push the shockwave away from the wings (or push the wings back away from the shockwave depending on your perspective).


text:
Why does it take months, sometimes years, for the BEA and NTSB to complete aviation accident investigations?
I'm aware that there is a lot that goes into an aircraft accident investigation, so I understand that the investigations take a long time for a reason.
But what I don't know is what those reasons are.  I'm hoping someone with some familiarity can give a high level overview as to why aircraft accident investigations take so much time?

have you ever fully read a report? collecting the information, validating it, performing analysis, setting up simulations to validate/disprove theories, interviewing experts, rinse and repeat until a valid explanation is found, drafting the reports and checking them for mistakes. Not exactly the kind of job you can take care of during breakfast.

@Federico Several, and I'm aware it takes a lot of time (I said so in the question), but I'm curious if someone who has done investigations can go over the process for us.

Also, realize both these organizations are small in the government scheme... I believe the NTSB is less than 300 people last I read.

That are small organizations are working on multiple cases with limited resources

It's may be worth pointing out that some investigations result in follow-up activities that continue long after the report is issued. The longest one that I'm aware of is still in progress after about 25 years. The evidence they are looking for (improving the detection of defects in the material used to manufacture certain components) flags up approximately one new data point per year - and of course the capabilities of the technology used in the process doesn't stand still for a few decades, while you are collecting a reasonable sized set of data points to analyze statistically.

Not so slow. AF447 accident in June 2009: The BEA released two interim reports on July and December 2009.  The recorders were found in May 2011, a third interim report was released in July 2011, and the final report on July 2012 (see page 19 and following for a recap of the activities).

When it comes down to it, accident investigations involve lots of coordination between many different groups, companies, and agencies, often around the world. These groups may range from helpful and responsive to completely uncooperative.

Visiting the Site - First there will be a team that heads to the incident site to collect information. If the site is easily accessible, this may be a fairly quick process. If the site is tough to access, or takes a while to find, this will of course take much longer. This is generally the part most visible to the public, but is only the tip of the iceberg in generating a useful report.
Research - There is also a lot of research that must go on behind the scenes. Especially if a possible cause is not clear, investigators must look into many factors to rule out the options. This can be a lot of information and may involved many different groups around the world:

Previous maintenance done on the plane
ATC and weather information about the flight
Information about the crew, the passengers, and the cargo on the flight
Many individuals may also be interviewed

If any of this initial information provides clues as to possible causes of an accident, they will also be gathering information about related aircraft design and functions. This can be a lot of information and may involve working with many different groups. The process of narrowing the focus of the investigation may involve tracking down different leads which end up being irrelevant. It may take a lot of effort just for the report to end up saying "X was not a factor."
Flight Recorders - Gathering the flight recorder data is not always as simple as plugging it in and reading the data. If there is damage, extracting and repairing the memory may take time. The FDR data must be analyzed and it may take additional work to investigate and verify the parameters. The CVR must be read out and then transcribed. Transcription is also a long process, involving different parties to the investigation, and must be synchronized to ATC and FDR information.
Analysis - Once the information has been collected, it must all be analyzed. Investigators may need to contact and work with many different groups and experts to develop an understanding of parts and processes that took groups of engineers years to analyze and design. Components may need to be analyzed down to a microscopic level. This is generally the longest part of the process. Investigators may realize that they must go gather more information. This may involve running tests on equipment to recreate issues. It may take a lot of work to not only explain what failed, but also the reasons behind the failure and how it could be avoided in the future. Most reports will find issues in many different areas, including human factors, regulations, procedures, and aircraft design.
The Report - The investigators will eventually need to come to some conclusions among themselves. This must all be written up in a report. Some reports can be quite lengthy. If you've ever had to write something like this, you might know that writing the report can easily take much longer than the work that went into it. There will also be an opportunity for parties to the investigation to make comments and provide feedback before the report is released. This is where the investigators get to share the results of all their hard work, and it will be published to get picked apart by the whole world, so it's worth taking the time to get it right.
Law - While most agencies like the NTSB focus on producing a factual report and not assigning blame, lawyers may also need to get involved. Investigations may involve dealing with proprietary or sensitive information, or groups that are reluctant to divulge information (see "completely uncooperative" above). Some accidents may involve criminal investigations as well, in which case organizations like the FBI may also be involved.
Followup - It's not even over after the report has been released. The agency will still need to follow up with regulatory agencies (like the FAA) about recommendations made in the reports and determine whether the responses are adequate.
Additional Duties - On top of all this, many investigators will not be working on one incident full time. Agencies like the BEA and NTSB investigate many smaller incidents as well. So imagine that at any given time, an agency may have many active investigations at different points in the above process. In the case of the NTSB, all of this is accomplished by about 400 people with a budget of around $100 million per year. While aviation safety is the largest part of this with about 130 employees, the NTSB also investigates highway, marine, and railroad/pipeline/hazardous material incidents. Employees also spend time on other things such as educating the public about safety issues, looking for ways to improve safety, and producing additional types of reports.

And don't forget politics - that can delay things esp. if there are international "sensitivities" (e.g., see EgyptAir Flight 990 where the Egyptians really disagreed with the NTSB investigation - the article doesn't say it delayed this particular investigation but the NTSB had to do followups for quite a long time, and you can see that the same problems might occur for other investigations which might delay them.)

Then there's good ol' beurocracy. Once they listened to the CVR on the Germanwings crash there was no arguing what took place. Yet it took almost a year to publish the report.

Every once in a while they also take all the wreckage and reconstruct the aircraft,  like TWA 800 which took considerable time to just recover off the sea floor.

I would argue with TomMcW about how simple the Germanwings 9525 investigation was.  Even when the root cause was obvious, the investigating authorities needed to determine a detailed description of the incident, evaluate ulterior motives, find out if the medical clearance procedures had been properly followed, if psychiatric practices took this outcome seriously enough, whether requiring two people in the cockpit would have helped, and detailed evaluation of the cockpit door operating procedures.  As fooot and kevin explain, the authorities leave no stone unturned when investigating.

I have not done any aviation investigation (I am far from even qualifying for the job), but I will try to relate it to something I have experience with: research paper writing.
A couple of similarities between both come to mind:

You don't know what you're looking for. You take a quick look at the big picture, pick your best guess (sometimes a wild guess), then dig deeper into that direction. If you're lucky, it works; usually it takes a few trials.
You need to call experts. Even within the field of aviation, there are many experts who specialize only in subfields, e.g. materials, human factors, aerodynamics etc. If you need to analyze the inner workings of a component or system, often times you have to contact the manufacturer. They have to then contact their employees, who may delegate that work to someone in their department, then that person has to pack up and travel. And it's likely you need many experts like these during the investigation.

If you suspect a bomb, you need an explosives expert.
If you suspect the weather was abnormal, you need a meteorologist.
If you suspect a structural failure, you need a materials expert.
If you suspect a defect in a component, you need an engineer from the manufacturer.
If you suspect pilot fatigue, you need a doctor who specializes in aviation.

You need to collect data. You need to conduct experiments or examinations to collect data. You need to design an experiment that fits your purpose. You need to set them up: apparatus, environment, users, everything. You need to do it scientifically. You need accurate measurements. Typical data I've seen in major accident reports include:

Flight data recorder / transcript of cockpit voice recorder. If the blackbox was damaged, technicians will need more time to extract data.
Flight history. When did it take off? Which airway it was flying? Do radar tracks deviate from the FDR? What was the weather?
Pilots' background. When did the captain get their license? When did he get training? Are there any failed checks in the past? Was the license actually valid and not fake?
Interviews. Did the previous crew have problems with this plane? What did the mechanics do? How did the ground crew fuel the plane? What do the pilots who landed just before this plane say? What do friends of the pilot involved think about his flying ability? What about the controllers?
Examinations. What was the position of the switches in the cockpit before impact? Did this debris come off because of metal fatigue? Are there signs of fuel leak? Did this break before impact, during impact, or during the post-crash fire?
Experiments. Would putting a rubber tire before the engine inlet lead to an explosion? Would the wings oscillate at 30mph crosswind? The manufacturer says the strength of the valve should withstand 100psi. Is that true?

You need to analyze. There is a huge amount of data. Do you find any anomalies? Any trends? Any correlation? Was that an experimental error, or did you find something?

Is the scenario reproducible in the simulator? This is common for control surface issues / corrupted flight data in the blackbox / abnormal cockpit warnings recorded.
Was it possible for the crew to save the plane? Quite often, for controlled-flight-into-terrain accidents, the report would state something like "if the pilot applied corrective action at or before the 13.7s mark, he would have cleared the trees by at least 200 feet".

You need to coordinate. Only the core investigators (those who get to decide the direction or conclusion of the investigation) have access to every piece of information. The others only get information related to their tasks. When necessary, the team would split up, e.g. two or three people investigate engine fire, another two or three study the charts and the flight path. You need meetings for the teams to present their findings and coordinate. Sometimes you would also need to coordinate with external parties:

If you need to run simulator trials, you need find someone with a simulator and volunteer pilots.
If you need aerial trials, you (likely) need special permission from the aviation authority and ATC, along with a plane and its pilots.
If you suspect the airline's training program is a factor, you need to talk to the chief instructor.

You need to prove. One of the reasons why NTSB reports are so reputable, is that they carry out extensive analysis to prove that every other possible scenario was not the case. They disprove every theory you can imagine, until you are left with nothing but the proposed cause of the incident.
You need to write. A decent research paper takes around a month just to write. You need to organize the content. You need to proofread for mistakes. A full investigation report is several times longer than a research paper.

care to explain the downvote?

This might be a slightly higher level than I was actually looking for... Steps 1, 5, 6 are kind of inherent to the situation.  Steps 2, I'd really like detail as to who (and why).  Step 3, I'd like more about what is collected and when it's considered "good enough" (since it will never be perfect).  Step 4, I wonder what methods are used, how do they use analysis to suss out more evidence and data?  How is that data shared?  How many people are involved?  Etc etc.  Just hoping for an answer that is probably the level of depth right below this one.

@JayCarr I've extended Steps 2, 3 & 4 based on what I've read in reports. I don't think there are specific methods for step 4, since you don't know what you're looking for. You really are just scanning all the data and compare that with expected values.

Very nice!  Switching my vote.  Between you and fooot, I think it's going to end up being whoever wins the vote war... Unless someone shows up and just blows me away or something.

Not an answer, but you might want to have a look at the investigation required as a result of British Airways Flight 38 where for a long time, there were indications as to the cause but actually replicating the issue to prove it took many attempts over many months before is was successful.

@DavidRicherby thanks...that was quite a lot of typos!

Because you do research, I wonder if you could speak to how difficult it is to coordinate that volume of information between hundreds of people? I think people forget that detail sometimes.  Many big projects, like this, have people dedicated to disseminating information (project managers).  I assume this is no different.

@kevin You're welcome. This proves your point 6 about why a good Stack Exchange answer can take months and sometimes years to write. :-)

@JayCarr Hundreds of people may get involved, but you don't have to share everything with everyone. Most people are either experiment participants or technicians, they're probably not interested in the entire scope anyway. The core members decide which data to extract and pass them onto different teams. If the data selection was bad, it is the core members' fault. That's why it is a role which requires experience & expertise.

RE the BA flight 38 investigation, a key quote from the Wikipedia page is "Certification requirements, with which the aircraft and engine fuel systems had to comply, did not take account of this phenomenon as the risk was unrecognised at that time." - so there would have been no "standard" testing procedures or facilities available that could reproduce the proposed cause of problem. Bear in mind that some of the tests with are performed "routinely" to certify engines may take a year to set up, run, and analyse the results, and you may get some feeling for the timescales involved.

If you would like a little "light-reading" to answer the question.  Take a look at the NTSB investigation manual as well as the 315 page appendix that goes with it.  Together they will give you some sense of the investigation process.  Just the time to read through that manual will take a while, let alone to perform the processes that it discusses.

Love how the appendix is over 5 times longer than the actual document...  I'll give this a look though, maybe I can write my own answer ;).

This answer is entirely correct, but it would be great if you could list or summarize some of the main activities from the manual in your answer, e.g. from the table of contents. As it is, this answer basically says "go read this document", and we usually try to avoid link-only answers here.

This is not a canonical answer to this Q as asked.
But I will add:

a slow investigation is not a bad thing, thorough is good
a completed investigation is just one part of the value in the holistic process of aviation safety
when investigators find something obviously wrong, corrective actions are sent out immediately to the aviation community in various directives. This often happens before an investigation is complete (I'm not an aviation professional, so someone may have good examples of this)
the most valuable part of this process is the accuracy of the investigation---and how good the corrective actions address the root cause of the accident


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Why don't NK-12 powered contra-rotating propellers turn in equal rpm?

The Kuznetsov NK-12 is a Soviet turboprop engine of the 1950s, designed by the Kuznetsov design bureau. The NK-12 drives two large four-bladed contra-rotating propellers, 5.6 m (18 ft) diameter (NK-12MA), and 6.2 m (20 ft) diameter (NK-12MV). It is the most powerful turboprop engine to enter service.
   (Wikipedia: Kuznetsov NK-12)

Tupolev TU-95 (NATO codename Bear) and Antonov AN-22 (NATO codename Cock) use the NK-12 turboprop engine, and as can easily be seen in many videos available on the internet, their propellers are not geared in a way that would make them turn in equal rpm.
TU-95 propellers turning ratio is such, that for each full turn of the front propeller, the one behind turns a little more than half a turn: TU-95 engine start on Youtube.

The two frames above show the aft propeller's lag after the forward propeller moved a quarter-turn.
For AN-22 the ratio is one full turn for front prop, and about 3/4 for the rear one: AN-22 engine start on Youtube.
Why such ratios? What is the reason the front propeller has significantly higher rpm compared to the one behind it? The air that has gone through the disk of the first propeller has obviously been accelerated, and has gained a swirl momentum. This would have to be taken into consideration somehow.
My own guess would be that this has to do with the aforementioned flow changes, load distribution between propellers, and/or something with vibrations and resonances of the powertrain.
Please do not hesitate to prove me wrong on this one, but for that to fly, I'd require some hard evidence concerning the NK-12 powertrain.

Can you link the document showing the gear ratios?

I do not have such documentation, the differing rotational speeds can be seen in the videos I've linked into the question. The AN-22 video is of a very good quality and shows the start of two engines, with the same ratio of propeller rpm on both. I'm sure there is documentation available, but most likely it is in russian, and that's a problem for me...

Ok, so you are basing this opinion of gear ratio solely on the videos, well, what if it's driven by separate turbines or a freewheeling planetary?

Well, the props are undeniably turning at different rates. I will be very happy if someone can prove either of what you came up with to be true. Separate turbines might be possible, and it has been speculated here on ASE. How would a freewheeling planetary gear transfer power by the way? (It might but I don't see how).

You have 1 power input (from the turbine) and 2 power outputs you are splitting power to. That's 3 things, and planetaries are really good at splitting power amongst 3 things. You just need to assure the loads are balanced. This is the same energy-balance problem that any car has with its differential.

I have to dig up a pic or a video of that, just cant wrap my head around it. In the mean time, you are clearly headed towards an acceptable answer here

The NK-12 engine uses a differential epicyclic (planetary) gearbox. Unlike the typical planetary gearboxes where one of the gears is fixed, here all three parts are moving: the turbine drives the sun, the planet pinion drives the front propeller, and the ring drives the rear propeller.
The gear ratios are the same between the front and rear, but the torque is not. The difference you see has to do with this torque difference, as well as with different rotating masses and friction during start. The torque is what is actually important.
If you hold one part, then the other two will be in the expected sync. Say, if you jam one prop and start the turbine, it will simply spin the other prop twice as fast. (But if you jam the turbine, things will be less intuitive: the props should turn in the same direction if you move one by hand). Now if you allow the turbine to free spin, you'll have two degrees of freedom, and the eventual relative RPM in such system will depend on the torque on each prop.
In the explanation below, I use this work by S. Falaleyev et al (pdf in Russian). Here is the diagram of the gearbox:

If we assume the torque balance such that the RPMs are the same (except the sign), the gear ratio will be (I skip the derivation):
$$i = 1 + 2(\frac{z_2 \cdot z_4}{z_1 \cdot z_3})$$
($z$ are the teeth numbers according to the diagram). For the given numbers we get $11.33$ (or rather, it should be $1/11.33 = 0.088$ reduction). The nominal speed of the props was just 735 RPM!
At the same time, the torque ratio (front to rear) is
$$\frac{M_f}{M_r} = -(1 + \frac{z_1 \cdot z_3}{z_2 \cdot z_4}) \approx -1.19$$
Thus, the front prop gets almost 20% more torque than the rear prop. This is why it spins up faster on start-up. (It is also lighter than the rear one).
At the nominal speed, the governor automatically sets a finer blade pitch on the rear prop to equalise the speeds. (Why not just accept higher speed/thrust from the front prop? Apparently, because the prop is limited by the tips speed, and we don't want the front one go supersonic while the rear one is relaxed).

Excellent answer!

This is great, before, with the help of google translate I was able to get some info about the gearbox but it was very general. Your linked document seems very informative, I'll have to dig in deeper! Very nice how they combined the reduction gear and the CRP divider into the same gearset.

Thanks for the document link, it's quite an interesting read. @Jpe61 let me know if you need help deciphering some passage.

brilliant! they use a differential gear as a natural torque splitter!

What had me fooled here was that I though there would be resemblance to cars and their differentials. Then there's the thing that the planetary gear on NK-12 is not your garden variety epicyclic...

I can't offer the technical backup, but years ago, I read that if the props are designed to run at slightly different rpms, it reduces the otherwise-hellacious sound produced by the contra props. Noise has been an issue with these since engineers started using them (late WWII I think).  Of course, none of the NK-12 powered aircraft are known to be "Whisper jets"...

This seems perfectly plausible.


text:
Can anyone identify the plane in this old photo?
It belonged to my uncle. May have been 1930s vintage.

It might help to say where in the world your uncle lived. Hopefully, somebody will figure this out.

If you think you know the answer, please write it as an answer not a comment.

I will say its odd how many planes in the late 20's early 30's looked alike.

It's probably a Stinson Junior. Built in the late 1920s and early 1930s.

Absolutely: some good pictures here: http://www.skytamer.com/Stinson_SM-8A.html. There is one picture from the same angle, and it's a 100% match.


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What is the difference between low energy warning and alpha floor in A320?
I am always confused between both of these two. I know both signals low speed or high AOA

Can't speak for the A320, but "low energy" usually refers to both low speed and power, like when you go to idle on landing.  Once the power comes off when you are at Vref you are in a low energy state.  AOA limits can be reached at a wide range of speeds and is not directly related to total energy state.

Low energy warning ("SPEED SPEED SPEED") is a function of Flight Augmentation Computer (FAC). It monitors aircraft’s flight path and warns if speed is about to decay below the threshold where power is needed in order to maintain positive (ie. climbing) flight path with pitch control.
Alpha floor is provided by auto thrust (A/THR) - (commanded by FAC), which sets full thrust (TOGA) regardless of pilot input if angle of attack rises above pre-determined angle. The threshold depends on aircraft configuration.

I would add that the Low Energy Warning is a warning like its name is referring to (Aural "SPEED SPEED") and the Alpha Floor is a protection, so it will take command on the ATHR and apply TOGA power if triggered.


text:
What are the reasons for not wearing polarized glasses during flying, if glass cockpit and polarized windshield aren't an issue?
So far the only reasons I can find are the newer planes come with polarized windshields that goes dark when you wear polarized glasses and the fact that glass cockpits become dark.
But is there a single reason to not wear polarized glasses while flying something like an vintage J3 Cub with nothing but some gauges etc? Can you see the PAPI lights and other necessary stuff? What keeps an pilot from wearing polarized glasses?

LC displays should be the only problem. Why are you asking in the first place? Have you been told not to wear polarizing glasses in the cockpit?

@bogl The multiple sources on the net says it keeps you from seeing the screens and breaks your vision when coupled with ann polarzied cockpit  glass and I wondered whetever there were other problems etc.

It's not the windows, it's the instruments.  I notice this even in my car now, tilting your  head and the two polarizations make screens go dark.  Need to find my RayBan unpolarized glasses now that I've gone to electronic screens for moving map GPS and attitude indicator. The old vacuum driven gyros didn't have that issue.

Glass cockpit actually refers to electronic displays, not to the wind shield. Possibly that is a source of confusion?

Been wearing them off and on for... well, several decades since I took my first lesson.  No problems.  But I don't really use anything with electronic displays.

I wear polarized sunglasses when flying, but I specifically chose my frames so the instrument panel would be mostly beneath the lenses instead of to them because it's a lot darker inside; I didn't even think about polarization. No problems with glass so far.

@StephenS did you ever think about wearing gradient glasses?

@bogl according to some video I watched. The windshield in a prop plane got distorted colors and darkness all over when filmed through polarized lenses and I first noticed LCD weirdness when I tried to watch TV with my sunglasses.

@JonathanIrons Never heard of that until now. I think I'd need bifocals for it to work since my near vision doesn't need correction (yet), so it's simpler to just get lenses I can see under for close/dark things. Works well in the car too. Not so good for walking around, but better than nothing.


text:
What is meant by a "congested area of a city/town/settlement" in FAR 103?
FAR 103.15 states:

No person may operate an ultralight vehicle over any congested area of a city, town, or settlement, or over any open air assembly of persons.

I'm wondering what is meant by "congested area".  It sounds like you can fly over a city, town or settlement so long as it's not congested but...how is that defined?

I could not find a definite definition of congested area.
As mentioned here:

[...] neither the FAA nor the NTSB has ever provided [...] a precise
  definition of [...] a "congested area." Rather, a "congested area" is
  determined on a case-by-case basis. According to the Board, "the
  determination must take into consideration all circumstances, not only
  the size of an area and the number of homes or structures, but, for
  example, whether the buildings are occupied or people are otherwise
  present, such as on roads."

The same is echoed here:

[For the definition of congested area,] FAA and the NTSB have opted
  for taking a "case-by-case" approach in determining how to apply
  certain terms.

AOPA states that:

Congested areas. "Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of
  1,000 feet above the highest obstacle within a horizontal radius of
  2,000 feet of the aircraft." The FAA does not define congested area in
  the FARs or in the Aeronautical Information Manual. Interpretations in
  low-flight enforcement cases are not consistent for purposes of
  drafting a precise definition. Such a determination is usually decided
  on a case-by-case basis, and in the cases that we've seen, congested
  has been interpreted rather broadly. For example, a highway with
  moderate traffic was found to be "congested," as was a seaside area
  where 200 to 300 persons were sitting on the beach or bathing in the
  water.

The same can be seen here, and here, and here.

This makes sense. After all a county fairground, 355 days a year may be an empty field... but for a few days...it's definitively congested.

Is it basically only being defined when there is an incident that is being investigated?  IE., aside from an obvious breach, you probably won't hear much about it unless there is some sort of complaint leveled against your flight path (or you have an accident, etc.)?

@JayCarr Not only in case of a breach. The AOPA definition (above) sort of answers your question. I think insurance companies might have more guidelines.

Huh, I guess the moral for ultralights is to just stay away from people as best as humanly possible.

How did a pilot measures 2000m AGL? Normally we measured ASL, as it normally done by any measurements.

It is not explicitly defined in the FAR Definitions and it seems the explicitly do not define it.
I would interpret (please keep in mind this is my interpretation and not law) a congested area/assembly of people, as anywhere you could not safely land the aircraft in the event of an emergency without endangering others and or yourself. This may better by explained with an example.
Lets say you are flying low over main street at rush hour, your engine goes and you are forced to make an emergency landing. You have limited distance and direction to go and can not safely land the aircraft with out hitting either a building, people or vehicles on the road thus endangering them.
With that being said, you are correct that class B generally lies around the big cities in this country, and other cities may reside in class C, you can still have a congested area or open assembly of people (concert, state fare etc) in class G airspace.

Actually, everything I'm reading says that the FAA very specifically does not have an explicit definition of "congested".  Not sure why, but they refuse to define it and, thus far, no real pattern has emerged from the rulings regarding the term.  So it remains emphatically undefined.

I provided my definition as what I feel a congested area is and what would be a safe bet to use when making your own decisions. I see that the FAA takes it on a case by case situation but that means that its up to you to define and you should build your own definition as best you can to avoid being in the situation where they need to investigate it.

I'd agree, and on the whole your answer adds a lot to the conversation, I just disagree with the second clause of your first sentence, that's all.  There is no explicit definition in this context.  But yes, you should do your best to create an internal definition to hopefully stay out of any situation where the FAAs definition (whatever it may be) actually matters.  And you've done that here.  It was surprisingly difficult to choose between the answers, I went with the other because of it's citations mainly.

Ahh i see what you are referencing, I was hesitant to say it was or was not defined when I researched it as I had not found anything but I will edit it to reflect what seems to be the case.

Awesome, and again, I really did appreciate your answer.  It's another one of those cases where I wish I could combine a couple answers into one...

While getting my Aerospace Engineering degree a "Regulations and Legal Interpretations" class was mandatory. As part of that we studied cases against pilots such as flying low over farm houses. The judges have been persuaded to accept 2 or more people as "Congested" or an "assembly" based on existing automobile traffic laws that fines people for violating speed limits for crowds or people (plural). For example, "when children (pl 2 or more) are present".
Take this at face value. I don't agree with it but it seems to be a case where the Government feels more successful by being vague then being reasonable. To me something like "more than 25 people in a 1 mile radius" would be more quantifiable.

If a regulatory authority does not define the word, then by default it is the common definition of the word. Un-congested vs congested. Think of the phrase Open space and occupied space. If flying over occupied homes or businesses it is by definition congested. If you are flying over open land it is un-congested. In reality, the FAA uses many words and phrases with out providing what their definition is and leaves it to the common understanding rather than a unique definition. Simple English...but not to those who push the limits.

You can fly over a city which is congested, by following railroad tracks, etc, those areas are uncontested.
But, you must be in class E or G airspace, unless you have clearance from the controlling agency.
The FAR 103.11(b) is only for flying with strobes, during civil  twilight. You must be in uncontrolled airspace, which is class G, generally, but not always, starting at 700 or 1200 feet AGL.
Where I live ..0-500 feet is class G and class B starts at 500' ago. So I could fly my ultralight at 400', 20 minutes after sunset, along railroad tracks thru a major city.
A bad decision is called pilot error, your job is to keep others safe from your stupidity. :)

Welcome to Aviation.SE! I removed the large image from your post, since it was not relevant to the answer. I was also wondering if you have a source that confirms that railroad tracks are not considered congested areas?


text:
Are taxiing aircraft to hold short of taxiway hold short markings if not instructed by ATC to do so?
At some airports, some taxiways have taxiway hold short markings (single dashed yellow lines against a black background). Do taxiing aircraft by default need to hold short of these markings if they're not instructed by ATC to do so?

Well, the first thing that enters my mind when I see this question is: What can happen if you don't, while ATC expects you to? What can happen if you do, while ATC expects you not to? How does the severity of these two compare?

@MichaelKjörling a) Maybe Bang Bang, b) Maybe Shouty Shouty ATC c) Bang Bang worse than Shouty Shouty

@Jamiec You just won the Internet.

@Jamiec, that's one of the best summaries ever.

Is there a specific country you are asking about?

This is one biggest pet-peeves of mine that even old, experienced pilots have missed after years of flying.
Taxi instructions are just that: Instructions, not clearances. Mind you there are some clearances when on the ground aswell, crossing a runway, entering a runway, backtracking, maybe some others.. mostly involving the runway. But a general text like ‘Taxi to holding point Charlie via Golf’ is an instruction.
Ok, so the difference is just pedantic but there is one big caveat: when you are cleared to land, cross, enter on a runway you are guaranteed the runway is, well, Clear and will continue to be so for the ammount of time you need it to land/take-off.
This is not true for taxi instructions. You are instructed on what path to take on the ground, but the taxiways and route to your runway is most definitelly not cleared of other aircraft (unless we are talking about low visibiliy ops, but just bear with me for now). When taxing you are still supposed to look out for other trafic and try to avoid hitting other airplanes taxing around as much as you can.
You might be instructed to go via one taxi path and then later told to give way to another aircraft comming from your left. See how the path has not been ‘cleared’? It is not a clearance.
All this talk to sum up: entering/crossing a runway is a clearance. Crossing a (non-mandatory) taxiway holding point is not. Is the answer slowly building up ?
Crossing a runway hold short makrking (two dased lines and two solid lines) you need clearance.
Crossing a taxiway holding marking (or intermediate holding position, or cat 2/3 holding when not in low-vis) does not require one.

Thanks. That definitely makes sense. While we're on the subject, does the same thing remain true for ILS critical areas? I'm pretty sure you don't have to remain clear of them unless instructed to do so. Is it a violation (e.g. could your PPL be suspended) if asked to remain clear, but you entered into one?

A taxi clearance from ATC to a specific point on the airport gives the pilot authorization to cross all hold short markings or hold position markings for intersecting taxiways up to the clearance limit of his taxi instructions without stopping. If a pilot is given a hold short clearance at a certain position, he must stop and ask ATC for permission to proceed. If there is no conflicting traffic as the pilot reaches the hold position, ATC will normally give prior authorization to cross that hold point before the aircraft reaches it. Any position markings that require the aircraft to hold will be given in the original taxi clearance and would have been repeated back when receiving the taxi clearance. If not previously cleared beyond a hold point, the pilot must STOP until further clearance is authorized.  Runway Holding Position Markings identify the locations on a taxiway where an aircraft MUST STOP when a clearance has not been issued to proceed onto the runway and no part of the aircraft can extend beyond the holding position marking. This is usually the last position of a taxi clearance limit and normally would be a change of frequencies for take-off authorization. One point to remember: if at any time you are in doubt of of your instructions,  STOP! Don’t be embarrassed to ask questions of the controller. It’s okay to ask for directions or for
progressive taxi instructions, if you are not familiar with the airport or route of taxi that has been assigned.

This is no longer true.  Each runway crossing must be called out separately: https://www.aopa.org/news-and-media/all-news/2010/may/13/coming-soon-new-taxi-procedures

Good point! I had interpenetrated this question to be taxi instructions for departure where more advanced precise planning can be made. Taxi instructions on arrival are more difficult to predict; hence, the need for more clarification from ATC to prevent  mistakes

Its not an issue of "clarification to prevent mistakes".  Your statement that "A taxi clearance from ATC to a specific point on the airport gives the pilot authorization to cross all hold short markings" is just wrong and a rules violation.  ATC can only clear you to past one hold-short at a time.

Obviously, we are not on the same page. A hold point can be issued to any position on the airport where ATC envisions a traffic concern  such as a truck crossing road or runway not in use where caution is advised. This is not the same as what you are stressing which is a Runway Holding Position. These consist of four yellow lines, two solid and two dashed,spaced six or twelve inches apart, and extending across the width of the taxiway or runway. The solid
lines are always on the side where the aircraft must hold. To cross a position so marked, a clearance is required before further taxi.


text:
What is the reason for this route while flying from USA over Cuba?
I was flying from Canada to Jamaica, and just past Florida the plane turned 90 degrees towards Nassau. It then continued over Cuba. Is this because of the situation between Cuba and the United States? It appears the plane must enter Cuban airspace from international airspace. But I have heard some planes can enter Cuba via corridors from the US. I am confused over this situation.
This was flight TS0373 on 17Jan2016.

Why the down-vote? Seems a good ATC question. Mark, do you remember the flight number?

When I flew from TEB to Kingston about 15 years ago in a PC-12, we used Jeppesen's international trip planning service to obtain all of the required overflight permits, make landing, customs and fueling arrangements, and provide  weather services.
The route we were provided by Jeppesen (which I don't remember exactly now) required airways in non-US airspace, and had specific entry and exit points in Cuban airspace, probably based on that country's radar and separation requirements.
It appears that OP's aircraft flew FOWEE-URUS-UCA-MLY
And as you can see from FlightAware, the KMIA-MKJP route often takes this routing.
So the question is not why did the "the plane turned 90 degrees", but rather why wasn't the plane vectored directly to FOWEE in the first place. The most likely reasons are weather and traffic.
.

And before JUNUS? EONNS and AVION or ELLEE for the horizontal leg of the screenshot. That would make sense if this is to avoid W-465 B as suggested by @Pondlife. Related area on SkyVector.

There are two large warning areas there, W-465 A and B. Both are active "intermittently" up to 70,000ft. It seems likely that the route was taken to avoid those areas.